1. Field of the Invention
The present invention relates to a pixel circuit for active matrix OLED and driving method which provide the illumination uniformity.
2. Description of the Related Art
Organic light emitting devices is a new light emitting technology, its principle is a sandwich structure that organic film to place in between two electrode layers. The light need transparent to device, so one of electrodes needs to use ITO electrode. When drive a forward bias to device between anode and cathode, the electron and hole that generated by anode and cathode will empty into light emitting material and then emit light by radiation and re-combine method.
The major application of Organic Light Emitting Devices is display, the pixel circuit is similar with the circuit of TFT LCD, they are all matrix arrays. An illustrated view showing a pixel circuit of Organic Light Emitting Devices of the prior art is shown as FIG. 1. After scan light 12 turn on the transistor 100, data line 10 provides a voltage and stores to capacitance 102. It's equal to voltage of transistor VGS, and transistor 101 convert voltage to current and current through transistor 101 by power line 11 and then transmit to Organic Light Emitting Diode. The current formula is
  I  =            1      2        ⁢                            k          ⁡                      (                                          V                                  G                  ⁢                                                                          ⁢                  S                                            -                              V                t                                      )                          2            .      The problem which pixel circuit of prior art is the threshold voltage of TFT has big variation. It causes big variation of current I, and different current of OLED in pixel circuit. Finally, the uniformity of illumination isn't well.
From FIG. 2 is an illustrated view showing a local pixel circuit layout on display panel of the prior art. If the voltage VDD of signal line 21 is 12V, then maintain wholly white frame need 8V that data line 22 writing a voltage. When the first scan line SN−1 scan and turn on, writing 8V to point A. Thus, the voltage on capacitance 23 is 4V and current generated by transistor M1 under VGS transmit to OLED 24, transmitting from transistor M1 to OLED through signal line 21. When the first scan line SN−1 cut-off and the second scan line SN turn on, the data writing 8V to point B and transistor M2 generate current through signal line 21, but point C is even lower than 12V because parasitic resistance of signal line 21 has IR-drop. It causes the voltage of capacitance 25 on pixel circuit P2 is not equal to voltage of capacitance 23 on pixel circuit P1, and the frame from top to bottom generates non-uniformity when writing the same data. This kind of phenomenon which parasitic resistance of signal line 21 to descend the voltage VDD is called IR-drop.
Refer to FIG. 3 is an illustrated view showing a pixel circuit of OLED of the another prior art. This circuit uses four Thin-film Transistors (TFT) 30,31,32,33 and two capacitance 36,37, wherein the value of capacitance for capacitance 36 is C1 and the value of capacitance for capacitance 37 is C2. Four transistors include drive transistor 30 which convert voltage to current and three transistors 31,32,33 which to do turn on or cut-off. Driving has two statement, one is AutoZero statement that using transistor 31,32 short, transistor 33 open and data line 34 transmits a VDD data, transistor 30 forms a connection of diode because transistor 32 short and point A stores the threshold voltage Vt1 of transistor 30. Another statement is writing statement that transistor 32 cut-off, data line 34 transmit a correct data and using capacitance couple principle, voltage of point A stores the value of
            Δ      ⁢                          ⁢      V      ×                        c          1                                      c            1                    +                      c            2                                +          V      t1        ,ΔV is the voltage volume of couple. When transistor 33 turns on, the voltage of point A lets transistor 30 generate current, the current formula is
      I    =                  1        2            ⁢                        k          ⁡                      (                                          V                                  G                  ⁢                                                                          ⁢                  S                                            -                              V                t                                      )                          2              ,the Vt in formula will be eliminated. The current has relationship with voltage on data line 34 and no relationship with the threshold voltage Vt of transistor. It can overcome the threshold voltage has variation induced current and illumination also has variation in former prior art. Due to this circuit need four transistors and two capacitance and need two statements, so also need two complex control signals.
Refer to FIG. 4 is an illustrated view showing a pixel circuit 4 of OLED of the another prior art. This pixel circuit 4 uses four Thin-film Transistors (TFT) 41,42,43,44 and one capacitance 45, wherein the function of transistor 41 is a switch, transistor 42 convert voltage to current and provide Organic light emitting diode (OLED) 46, and the function of transistor 43,44 is compensating threshold voltage (Vt) of transistor 42. Thus, scan signal SN turn on transistor 41, data line 47 provide a lowest voltage, and then transistor 44 will turn on and decrease voltage of B point to turn on transistor 43, data line 47 provide higher voltage VDATA. Due to low voltage of B point will turn on transistor 43, thus, providing the current of OLED 46, the formula is
                                          I            ⁢                                                  ⁢            d                    =                      k            ⁡                          (                                                V                                      G                    ⁢                                                                                  ⁢                    S                                                  -                                  V                  t                                            )                                      ,                                  ⁢                  k          =                                    1              2                        ⁢                          μ              ·                              C                OX                                      ⁢                                          W                L                            .                                                          (        1        )            VG42=VB=VA−Vt43  (2)Id=k(VDD−(VA−Vt43)−Vt42)2  (3)
In formula (3), Vt43=Vt42 because the difference is close between transistor 42 and transistor 43, and process variation small. It replaces to formula (2) is Id=k(VDD−VA)2, VA=VDATA, it shows no relationship with current and threshold voltage Vth of transistor.
In formula (3), VG42 is a voltage of gate of transistor 42; Vt43 is a threshold voltage of transistor 43; Vt42 is a threshold voltage of transistor 42; VDD is a voltage transmitted by signal line 48.
From the result of formula mention above, this circuit 4 can overcome threshold voltage variation of transistor on display induced illumination non-uniformity and layout area is smaller. But before writing a real data, it need provide a low voltage and then transistor 42 provide a high current to OLED 46, the illumination of display will brighter first and recover to normal status. It causes shorten the life-time of OLED and worse image quality, and operation complex because it need to provide a low voltage before writing correct data in data driving circuit.
To resolve problems mentioned above that threshold voltage and IR-drop induced illumination non-uniformity of OLED. In this invention propose a pixel circuit for active matrix OLED and driving method and achieve the purpose of the illumination uniformity in display.